The main goal of this proposal is to answer the question why two typeII DNA topoisomerases with seemingly identical enzymatic properties are both needed in mammalian development. Under this central theme there are three specific aims. Aim I addresses the hypothesis that the activities are interchangeable but differences in temporal and/or tissue specific regulation of the topoisomerase II alpha and beta genes are responsible for the necessity of both. Aim II is directed at answering the question raised by the recent finding that inactivation of the beta isoform in the mouse prevents normal axon growth. A reporter gene (GFP) will be fused to topoisomerase II beta to allow detailed monitoring of this isoform during development. Finally, aim III addresses the hypothesis that the distinct C-terminal domain of the isoforms are responsible for their indispensability. This hypothesis will be tested through swapping of the C-terminal domain of one isoform by another.